1. Field of the Invention
The present invention relates to a bidirectional communication system employing a cable, such as for CATV (cable television), and in particular to an upstream communication noise level determination method for a bidirectional communication system.
2. Related Arts
FIG. 7 is a diagram illustrating the basic arrangement for CATV (cable television), which is an example bidirectional communication system. In FIG. 7, the CATV system comprises: a center unit 10, which is a CATV station; terminals 20, provided for a plurality of CATV subscribers; and a transmission system 30, for connecting the center unit 10 and the terminals 20. The transmission system 30 includes a transmission path 31 constituted, for example, by an optical fiber or a coaxial cable, and a repeater 32 for amplifying a signal sent across the transmission path. When an optical fiber is used as one part of the transmission path 31, a photoelectric converter is connected to each of its ends.
FIG. 8 is a diagram illustrating an example arrangement for the repeater 32. The repeater 32 comprises a trunk amplifier (TA), a trunk distribution amplifier (TDA), a trunk branch amplifier (TBA), a branch amplifier (BA), an extension amplifier (EA), and distributors (tap-offs: TOs). The function of the amplifiers is the amplification of signals transmitted across the transmission path 31, and the function of the tap-offs is the diversion of signals on the transmission path 31 to the terminals 20 of the subscribers.
In the arrangement shown in FIGS. 7 and 8, for example, a band of 70 MHz to 770 MHz is employed as the transmission band for downstream communication from the center unit 10 to the terminals 20, and a band of 10 MHz to 50 MHz is employed as the transmission band for upstream communication from the terminals 20 to the center unit 10. Normally, for allocation purposes, the television signal channels are divided by bands of 6 MHz, one of which is provided for each channel.
For the downstream communication stage of conventional bidirectional communication, a signal having a predetermined level is transmitted from the center unit 10 to the terminals 20. For the upstream communication stage, the transmission level at the terminals 20 is adjusted so as to maintain a constant reception level at the center unit 10. Generally, the signal level is attenuated by a transmission loss across the transmission path 31 and the repeater 32. The level of the transmission loss differs with the lengths of the transmission paths 31, which extend from the individual terminals 20 to the center unit 10, or with the number of the repeaters which are provided. Therefore, during the upstream communication stage the transmission signal levels at the individual terminals 20 vary in accordance with the level of the transmission loss in order to maintain a constant reception level at the center unit 10.
In addition, in order to maintain a constant signal communication quality, signals received by the center unit 10 and by the terminals 20 must satisfy a predetermined C/N value which is a ratio of a signal carrier level to a noise level.
FIG. 9 is a specific diagram showing a carrier level and a noise level. For examples (A) and (B) in FIG. 9, the horizontal axis represents a frequency and the vertical axis represents a level. For examples (C) and (D) in FIG. 9, the horizontal axis represents time and the vertical axis represents a level. When, as is shown in example (A), the noise level is low relative to the carrier level (the C/N value is high), almost no noise element appears in the carrier, as is shown in example (C).
However, when, as is shown in (B), the noise level is high relative to the carrier level (the C/N value is low), the noise element which appears in the carrier as is shown in (D) is regarded as a signal element, which may cause a demodulation error and result in data being obtained that differ from the original data. As the noise level is high, the noise element is increased and the probability that a conversion error will occur is also increased.
The noise which occurs in the transmission system 30, such as along the transmission path 31 and the repeater 32, is transmitted with a downstream signal to the terminals 20 during the downstream communication stage, or is transmitted with an upstream signal to the center unit 10 during the upstream communication stage. In the downstream communication stage, since the center unit 10 and the terminals 20 have a one-to-one correspondence and only one route is provided for the downstream signal for each terminal 20, the noise carried by the downstream signal received by the terminals 20 is not increased.
In the upstream communication stage, however, since the upstream signals from the terminals 20 are collectively transmitted to the center unit 10, the noise levels of the individual signals are superimposed (joined), and the final transmitted noise level is increased considerably. This noise is, for example, noise which is produced by the terminals 20 and the repeater 32 themselves, or an external noise resulting from the inadequate securing of a connector (not shown) for connecting the transmission path 31 and the repeater 32, or by the loosening of a connector that may occur as time elapses. Various noise factors are present in the communication system.
FIG. 10 is a conceptual diagram of the convergence of noise during the upstream communication stage. In FIG. 10, upstream signals (a, b, c and d) transmitted by the individual terminals 20 are synthesized when they pass through a repeater 32, which has a branch function, and a synthesized upstream signal (a+b+c+d) is obtained which is carried along the transmission path 31 and is finally received by the center unit 10. The noise levels carried by the upstream signals (a, b, c and d) are superimposed on each other when the upstream signals are synthesized.
As a result, when the upstream signal is received by the center unit 10 in the upstream communication stage, the C/N value is considerably greater than the C/N value of the downstream signal received by each terminal 20 in the downstream communication stage. Therefore, in order to maintain a constant or higher quality for the upstream signal which is received by the center unit 10 in the upstream communication stage, the levels for the upstream signals transmitted by the terminals 20 must be set by measuring the noise levels, so that the C/N value of the signals is equal to or higher than a predetermined value.
Furthermore, since more transmission paths 31 are required as the number of CATV subscribers is increased and the communication service is expanded, or since the transmission path 31 and the repeater 32 may deteriorate due to the elapse of time, the noise level of the signal is increased and the quality of the signal is deteriorated.
Conventionally, however, the noise level included in the signal in the upstream communication stage is not measured, and no countermeasure is employed for the time-transient deterioration of the noise level.